It's good to be playful in relationships which it seems like you two are. He probably does like the contact with you and making you laugh. He equates that with making you happy (which it does). As for grabbing your bum - it's meant in a flattering way by guys most of the time. I don't get it, personally I hate if a guy slaps or grabs my ass, but it's meant mostly to be a playful or intimate guesture.
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